<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body{
      margin: 0;
      padding: 0;
      background-image: url(../images/grid.png);
    }
    canvas{
      background-color: rgba(255, 0, 0, 0.1);
    }
  </style>
</head>
<body>

  <canvas id="tutorial" width="300" height="300px">
    你的浏览器不兼容Canvas,请升级您的浏览器!
  </canvas>

  <script>
    window.onload = function() {
      let canvasEl = document.getElementById('tutorial')
      if(!canvasEl.getContext){
        return
      }
      let ctx = canvasEl.getContext('2d') // 2d | webgl

      requestAnimationFrame(draw)

      /**
       这个函数就是动画的一帧
       // 现在这个函数在1秒钟会回调 61 次
      */
      function draw() {
        let second = new Date().getSeconds()
        console.log('draw')
        ctx.clearRect(0, 0, 300, 300)
        ctx.save()

        // 1.开始绘图
        ctx.translate(100, 100)
        ctx.rotate(Math.PI * 2 / 60 * second)
        ctx.lineWidth = 6
        ctx.lineCap = 'round'
        ctx.strokeStyle = 'red'

        ctx.beginPath()
        ctx.moveTo(0, 0)
        ctx.lineTo(0, -80)
        ctx.stroke()

        ctx.restore()
        requestAnimationFrame(draw)
      }

    }
  </script>
</body>
</html>